It is known that the occurrence, during the flight of a transport plane, of a failure of one of its engines demands that the flight crew react quickly in order to ensure its safety in the short-term (control of the thrust control and of the instantaneous trajectory), then define the procedure to be followed in the longer term. The latter may consist in:                continuing the flight, if necessary at a lower altitude; or        returning to the departure airport; or        diverting to an alternate airport.        
The operations control center of each airline prepares a flight folder for each flight. This flight folder could include fall-back trajectories in case of engine failure. Such a fall-back trajectory is actually obligatory for air operations referred to as “ETOPS” (Extended Twin-engine Operations) that cover twin-engine flights of which at least a portion is at over one hour flying time from any airport.
When an engine failure occurs, in addition to dealing with the failure itself (shutdown defective engine, etc.), the flight crew must carry out a certain number of relatively complex tasks:                adapt the thrust on the engine or engines that remain available;        communicate with the air traffic controllers;        define the destination (which could be a new one) and input it into the flight management processor, for example of the FMS (Flight Management System) type, together with the trajectory for flying to this destination;        define the flight strategy in the vertical plane according to the environment, in other words:                    choose the climb or descent slope, then the new flight level;            choose the new speed profile; and                        ensure that the aircraft performance parameters (which, with one less engine, systematically over-consume fuel) will allow the chosen destination to be reached.        
As a result, the occurrence of an engine failure leads to a significant increase in the workload of the flight crew of the plane.
A system is known from the document FR-2 789 500 that comprises means capable of detecting any engine failure of a multi-engine aircraft and that allows the yaw control of the aircraft to be improved when an engine failure is detected. This system is based on the enhancement of an electrical flight control system described in the document FR-2 617 120.
It also known that many flights go through portions of airspace for which the notion of invariable airway does not apply. This is in particular the case above oceanic regions, for which substantially parallel routes are constructed by countries and defined on a daily basis as a function of the weather conditions and of the traffic requests presented by the airlines by means of a system called OTS (Organized Track System). For each of these routes, the aircraft may be separated:                vertically: every 2000 feet (around 600 meters) for example; and        longitudinally (for the same flight level): generally every 10 minutes as a minimum.        
When an engine failure occurs while the aircraft is situated on one of these routes which are, in general, laterally separated by 60 nautical miles (around 110 km), it is recommended to move away reasonably quickly from the planned route. The reason for this is that, owing to the loss of altitude induced by the engine failure, there is a non-negligible risk of coming into conflict with an aircraft flying under this route. It is generally accepted that the best strategy consists in turning by 90° and taking the aircraft to an intermediate lateral route.
The occurrence of an in-flight engine failure therefore constitutes one of the most critical cases in aeronautics. It very often causes anxiety and stress for the pilots, in addition to a significant increase in the workload, as previously indicated. On top of that must be added the fact that the aircraft must clear obstacles due to mountainous terrain (more critical because the flight ceiling decreases owing to the engine failure) and that, in the case where only one engine remains in operation, draconian regulations limit the flight time (for single-engine operation).
The procedures to be implemented by the crew in such a situation are therefore time-consuming and complex, and as a result open the door, in a stressful situation, to errors, confusion, forgetfulness, etc.